Current Conditions
Temp7.0 C
RH28 %
WindN 12 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

208
FXHW60 PHFO 272006
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1006 AM HST FRI MAY 27 2016

.SYNOPSIS...
A trough northwest of the state keeps winds light and from the
east-southeast through the weekend. Onshore sea breezes during
the day will lead to afternoon clouds and showers across interior
and leeward areas. Offshore land breezes at night will clear out
much of the area, but weak east to southeast winds will keep some
clouds across windward sections. A shower band will move south
over the islands late monday into tuesday, with a brief return to
trades behind it.

&&

.DISCUSSION...
Water vapor imagery shows a broad upper level trough west of the
state, with a west-southwesterly jet over the islands. At the
surface, high pressure is located far to the northeast, with a
trough a few hundred miles northwest of Kauai. 12z soundings show
the difference in airmass across the state, with inversions
ranging from around 8kft at Hilo to a weak one near 14kft at
Lihue. Precipitable waters are also 1.3 and 1.6 inches
respectively. Early morning MIMIC total precipitable water imagery
show similar values, with the drier air extending upstream to the
northeast from Oahu to the Big Island, but greater moisture just
lee of these islands extending to the northwest across Kauai.

Heavy showers flared up late last night within the moisture axis
over Kauai, with a few gages on Kauai reporting over 1 inch in the
past 12 hours and the Mount Waialeale gage measuring over 3
inches. Shower activity has died down this morning, but will flare
up again during the late morning/afternoon. Given the moisture and
instability, the heaviest showers are expected across Kauai.
Because of the light winds, sea breezes will again develop across
leeward sections. High-resolution guidance from the WRF ARW and
NMM runs shows activity greatest across Kauai and Oahu, but with
afternoon showers across all islands. The main focus will also be
across leeward coastal sections, where convergence between the
sea breezes and the weak background flow will be greatest. Have
updated the forecast to reflect the higher rainfall chances across
these areas for today.

Gradual stabilization is expected through early next week as
heights slowly rebuild over the state. The wind and weather
pattern we see today will continue at least for Saturday and
Sunday, with afternoon showers interior and leeward and some
lingering showers windward overnight. (Although the pattern won't
be exactly the same; it will be improving on Kauai, similar to
what the other islands will see today). Monday may also fall into
the same pattern as the rest of the weekend, but there is the
potential for greater shower coverage. Differences emerge among
the extended models with respect to an old frontal boundary/cloud
band this is expected to reach the area early next week. The GFS
is fastest bringing this band to Kauai/Oahu by early Monday, then
continue on to the Big Island on Tuesday. Other extended models
including the ECMWF and NAM have been slower, not bringing the
boundary this far south until late Monday/Monday night. The trend
has been faster though (as seen between the 00z and 12z ECMWF
runs), so this bears watching. If the faster solution pans out, we
would see higher rainfall chances for Memorial Day.

The other accompanying impact from this old front/cloud band will
be an increase in northeasterly winds with it. A new surface high
will setup along 30N, providing at least a few days of trade winds
during the course of the new work week. At some point another
system passing to our north may weaken and display the high,
cutting off the trades and returning us to another light wind
pattern. However, both the 12z GFS and ECMWF has backed off on
this next system, keeping us with a trade pattern for a couple
days longer. Given the time of year, would expect this trend
toward a more entrenched high to continue.

&&

.AVIATION...
A large cluster of low clouds just south of Kauai and moving north
northwest may produce more MVFR and isolated IFR conditions over
Kauai again by midday and into much of this afternoon. The
southeast wind flow across the state will tend to allow clouds and
showers to form during the daytime over mainly interior sections
of the islands. With the Big Island blocking much of the wind for
the other islands, expect to see daytime sea breezes and nighttime
land breezes for much of the state today and Saturday.

Will keep AIRMET Sierra in effect even though clouds have
temporarily diminished over kauai since more should reform in the
next hour or two as the clouds and moisture from the south move
up.

&&

.MARINE...
Winds and combined seas will remain below small craft advisory
criteria through the forecast period.

The current southwest swell will gradually diminish through the
holiday weekend. A small northwest swell arriving Sunday will
peak Monday well below the advisory threshold. See the the
Collaborative Near-shore Swell and Wind Forecast (SRDHFO) for
details on these swells. Otherwise rather small surf is expected.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

Discussion/Marine...Bravender
Aviation...Fujii

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office